Acetaminophen (APAP) is the most commonly used drug for the treatment of pain and fever in the world today. In large doses, APAP causes acute liver failure and is the leading cause of acute liver failure in the US. The current laboratory test for the diagnosis of APAP toxicity, measurement of APAP levels in peripheral blood, is only effective in the first 24 hours of APAP overdose. Acetaminophen Toxicity Diagnostics (ATD), LLC has developed a rapid, point-of-care diagnostic assay (dipstick) for the measurement of APAP protein adducts. Adducts are sensitive and specific biomarkers of APAP hepatotoxicity that may be detected in peripheral blood 6 to 7 days after a toxic overdose of APAP. The dipstick detects APAP irreversibly bound to proteins (APAP protein adducts) by using antibodies specific for adducts in the detection system. The dipstick is reliable, accurate and provides rapid results. In Specific Aim 1, the dipstick will be calibrated to detect levels of APAP protein adducts corresponding to severe liver toxicity and 2000 precision manufactured, research use dipsticks will be produced. In Specific Aim 2, the dipsticks will be tested internally for stability, specificity, and diagnostic sensitivity using stored, frozen clinical samples. In Specific Aim 3, six major hepatology centers will participate in a study to measure dipstick performance in patients with acute liver injury and acute liver failure. Successful completion of these aims will establish the clinical utility of the dipstick, an innovation that will enhance the diagnosis of APAP toxicity.